Are the Spartans an overrated force in history? Do we have many concrete examples of their hoplite formations superiority over other Greek polis?

They’re overated in the sense that they were an extremely brittle force, not capable of rebounding from defeats. 

But while they might have been

prōtos metaxỳ ísōn, the Spartans could indeed be beaten by other Greek polis. The Sacred Band of Thebes, for example.